1. Field of the Invention
The present invention relates to a beam scanning printer that makes a hard copy of a full-color image representative of an image signal for TV or an electronic image, by projecting scanning beam spots of different colors onto an advancing sheet of photosensitive material, wherein the beam spots of the different colors are formed at the same position in the same size per each pixel.
2. Background Arts
JPA Nos. 52-52598 and 55-144264 and many other prior art materials disclose those optical systems which record an image line by line on a photosensitive material, such as a photosensitive belt or a photosensitive drum, by scanning a laser beam across the photosensitive material through a rotary polygonal mirror. While the laser beam is scanned, the photosensitive material is moved by a mechanism in a transverse direction to the scanning direction, so a two-dimensional image is formed on the photographic material.
JPA No. 58-192015 discloses an optical system, wherein a plurality of light beams projected from a plurality of light sources are scanned through a common rotary polygonal mirror, so a collimator lens is allocated to each of the light sources, to produce parallel rays. The parallel rays are reflected from mirrors so as to position the optical axes of the light sources.
U.S. Pat. No. 4,641,950 published in 1984 discloses a scanning device for optically scanning a light beam along a scanning line to record a line of dots on a photographic sheet, wherein a plate with a slit is placed near the recording surface of the photographic sheet, to shape the scanning line in its widthwise direction.
U.S. Pat. No. 4,800,400 published in 1989 discloses a beam scanning printer that uses light beams of three primary colors, i.e. red, green and blue, from light emitting diodes (LED) for scanning and exposing an instant photographic film. In this beam scanning printer, the red, green and blue LEDs are arranged horizontally at give distance from each other in a light source section. The output light beams from the LEDs travel along different optical axes, and the optical axes are crossed on the recording surface of the instant photographic film, to form a common beam spot to the three colors on the instant photographic film. Thus, three color dots of one pixel are concurrently recorded on the instant photographic film. Through reciprocating movement of a mirror, the common beam spot is scanned across the entire width of the instant photographic film in opposite directions for main scanning as the instant photographic film is moved in a sub scanning direction transverse to the main scanning direction. During the scanning, peak values of the currents supplied to the LEDs are controlled at a high frequency in accordance with data of three color densities of each of many pixels to be recorded sequentially along the scanning lines.
For recording different color dots of one pixel by projecting a beam spot of each color onto a photographic material, it is necessary to form the beam spots of different colors at exactly the same position for each pixel in exactly the same size with respect to all pixels throughout the scanning lines. For this purpose, it is possible to provide a separate lens system for a respective color beam to cross the different optical axes of the different color beams on the recording surface to form a common beam spot. However, this solution would be expensive and need a larger space. On the contrary, to cross the optical axes of the different color beams through a single lens, some of the optical axes should be directed to the single lens in aslant to the optical axis of that lens. In that case, it is very difficult to control the optical axes so as to make the size and the projecting position of the different color beam spots coincident with each other. It is also hard to use an aspherical lens in order to eliminate chromatic aberration where the optical axes of the beams slant to the lens optical axis.
In view of the foregoing, an object of the present invention is to provide a beam scanning printer that makes it easy to control size and position of beam spots of different colors on a photographic material, that is preferable for miniaturizing and lightening the whole optical system, and that makes it possible to cut the cost of manufacturing.
To achieve the above object, the present invention provides a beam scanning printer that comprises a plurality of light sources that output rays of different colors from each other at individually controllable timing and intensity; a light mixing device for mixing the output rays from the light sources to produce a mixed beam of the different colors with a common optical axis; a converging optical system for converging the mixed beam of the different colors to form a beam spot on a photosensitive material; a scanning device for optically scanning the beam spot across the photosensitive material to provide a scanning line; a device for moving the photosensitive material relative to the scanning device in a direction transverse to the scanning line while holding the photosensitive material in a plane including the scanning line; and a control device for modulating the output rays from the respective light sources in accordance with three color densities of many pixels to be recorded along the scanning lines in synchronism with the scanning of the beam spot and the relative movement of the photosensitive material.
Since the light beams of the different colors are mixed so as to have a common optical axis before being converged, the light beams may be converged at a common focal point with the same diameter through the converging optical system. Because of the common optical axis, lens elements, that constitute the light mixing device, the converging optical system and the scanning device, may have smaller diameters, and also the requisite number of lenses is reduced in total, as compared to a case where the light beams of the different colors are directed along different optical axes to these lenses. Therefore, the beam scanning printer of the present invention gets a large degree of freedom in arrangement of the respective elements, and cuts the cost of design and manufacture. Also the beam scanning printer can be light and small. Because the position and the size of the beam spots of the different colors coincide with each other, a resultant printed image is superior in color definition, gradations, grain texture, and reproduction.
In a preferred embodiment, the output rays radiated from the light sources are converted into parallel rays through collimator lenses placed in front of the respective light sources, and then optical axes of parallel beams of the different colors are aligned through selective reflection surfaces. According to this configuration, it is unnecessary to equalize optical path lengths of the different color light beams. It is not always necessary to make the respective color light beams completely parallel, but they may be slightly radiating on the long wavelength side, or convergent on the short wavelength side, in order to compensate for chromatic aberration of the converging optical system.
According to another embodiment, the radiating beams radiated from the light sources are mixed with each other through a light converging member. The radiating beams are directed to a large-diameter incident surface of the light convergent member, and projected from a small exit surface thereof. The diameter of the exit surface is so small that may be considered to be a point. As being projected from the point, the mixed radiating beam from the light converging member is converted into a substantially complete parallel beam through an appropriate collimator lens. The substantially complete parallel beam of the three colors may be converged through a simple convergent lens to be a common beam spot at a common focal point of the convergent lens by locating the convergent lens at any position on the optical path of the parallel beam.
Using LEDs as the light sources is preferable because a low voltage power source such as a dry cell is enough for driving the LEDs, and their outputs are controllable with high accuracy, speed and fidelity through a simple current control circuit.